1. Field of the Invention
This invention relates to superconducting materials and to a method of preparing such materials by epitaxial growth.
2. Description of the Prior Art
Superconductivity, a thermodynamic state characterized by zero electrical resistivity and perfect diamagnetism, was discovered in 1911 by Kammerlingh Onnes at Leiden University when he found that the resistance of mercury vanished below 4.2 degrees K. Although it was subsequently found that numerous other elements, alloys and compounds are superconductors when cooled to sufficiently low temperatures, it was also discovered that, even if the temperature was below the critical or transition temperature, superconducting properties could be destroyed by a sufficiently strong magnetic field which might be either an external field or one associated with the superconducting current. Commercial interest in the first discovered superconducting materials soon waned because even moderate magnetic fields destroyed the superconducting properties and superconducting properties were thus limited to low magnetic fields and currents.
Upon the discovery of what is now referred to as hard, or Type II superconductivity, the entire field advanced and revived commercial interest. Type II superconductors are often characterized by, among other anomalous properties, high transition temperatures and high critical magnetic fields and although they may have any one of several crystal structures, they are best exemplified by the A15 or beta-wolfram structure. The discovery of Nb.sub.3 Sn by Matthias, Geballe, Geller and Corenzwit in 1954 and the further discovery by Kunzler and coworkers of its unexpectedly high critical magnetic field established benchmarks which are still typical of the commercially promising high T.sub.c superconductors and are easily sufficient to justify the extensive studies during recent years directed toward commercial exploitation of superconductors. Commercial uses suggested include very high magnetic field devices used as magnetic containers for high temperature plasmas in fusion power sources or as magnets for bubble chambers or high energy elementary particle accelerators. Although much effort was directed towards studying Nb.sub.3 Sn, investigation of other materials having the A15 structure also proceeded.
Possibly the most significant advance in the latter respect was the discovery by Matthias and Geballe and their coworkers, U.S. Pat. No. 3,506,940, that an alloy of Nb.sub.3 Ge and Nb.sub.3 Al had a transition temperature higher than any known at that time including Nb.sub.3 Sn. This tended to support the thesis that Nb.sub.3 Ge is probably, from the standpoint of the parameters of concern, the most valuable of the studied A15 materials. Based on this and other studies which indicated that the maximum transition temperature of Nb.sub.3 Ge would occur at the stoichiometric composition, it was generally accepted that if Nb.sub.3 Ge were available as a pure A15 material, it would supersede Nb.sub.3 Sn for many purposes. Efforts made to produce Nb.sub.3 Ge in the desired structure have resulted in advances and identification of directions for fruitful exploration.
For example, a metastable A15 phase in Nb.sub.x Ge.sub.y films has been prepared by electron beam codeposition at an oxygen partial pressure of 2.times.10.sup.-6 Torr on a substrate held at 775 degrees C. during deposition. This technique extends the A15 phase boundary to about 23 atomic percent Ge. The A15 Nb.sub.3 Ge structure obtained, competing with the stable Nb.sub.5 Ge.sub.3, is, however, extremely sensitive to nucleation conditions, contamination, deposition rate and the substrate temperature. Thermodynamic considerations that favor formation of phases other than the A15 at the stoichiometric composition are the primary reason for the lack of success in preparing Nb.sub.3 Ge in a pure A15 structure at the stoichiometric composition.
It was also generally accepted that other materials, such as Nb.sub.3 Al and Nb.sub.3 Si, would have their maximum transition temperatures, and have increased commercial value, if they could be prepared in a pure A15 phase at the stoichiometric composition. Thermodynamic considerations similar to those described for Nb.sub.3 Ge cause phases other than the A15 to also be stable at the stoichiometric composition and have prevented preparation of the desired materials.